Recently, renewable energy such as solar energy and wind energy has attracted attention as a method to suppress greenhouse gas emissions, which are a major cause of global warming. However, renewable energy is heavily influenced by location and natural conditions. Moreover, since the output fluctuation is large, it is impossible to uniformly supply the energy continuously. Therefore, in order to use renewable energy for home or commercial use, a system that stores energy when the output is high and uses stored energy when the output is low has been introduced and used.
As such an energy storage system, a large-capacity secondary battery is used. For example, a large-capacity secondary battery storage system has been introduced into a large-scale solar power generation and wind power generation complexes. The secondary battery for storing a large amount of electric power includes a lead-acid battery, a sodium-sulfur (NaS) battery, and a redox flow battery (RFB).
Although the lead-acid battery is widely used in comparison with other batteries, it has disadvantages such as low efficiency, high maintenance cost due to periodic replacement, and generation of industrial waste generated when replacing the battery disposed. Although the NaS battery has high energy efficiency, the NaS battery operates at a high temperature of 300° C. or higher, which is problematic. The redox flow battery is capable of operating at room temperature and a capacity and output thereof can be designed independently whereby much research has been conducted recently for a large capacity secondary battery.
The redox flow battery is configured with a stack in which a membrane, an electrode, and a bipolar plate are repeatedly arranged in a series, similar to a fuel cell battery such that the redox flow battery is a secondary battery that can be charged with and discharge electric energy. In the redox flow battery, a positive electrolyte and a negative electrolyte, respectively supplied from the positive and negative electrolyte storage tanks of each side of the membrane, circulate to perform ion exchange and electrons move in above process to charge and discharge. The redox flow battery is known to be most suitable for energy storage system (ESS) because the redox flow battery has long life span and can be manufactured as medium and large sized systems of a kW to MW class.
However, the redox flow battery has a structure in which the tanks for storing a positive electrolyte and a negative electrolyte are disposed with a certain space (for example, a structure in which the tanks are disposed with a certain space on each side of the stack or below the stack). There is a disadvantage with respect to overall system volume, in which electrolyte circulation pipes connect the stack and the tanks to each other, compared to other power storage devices based on similar power storage capacities such as the lead-acid battery, a lithium ion battery, and the lithium-sulfur battery.
In addition, since a plurality of electrolyte circulation pipes are required to connect the stack, the pump, and the electrolyte tank, a pump capacity exceeding a predetermined standard is required in order to supply the electrolyte to each stack uniformly. As a length of the electrolyte circulation pipe becomes longer, the required capacity of the pump is increased such that a size of the pump and the manufacturing cost of the battery are increased, and an overall power efficiency is lowered due to the increase of the power consumption due to the increase of the pump capacity.
In addition, a general battery should have a high responsiveness in charging and discharging operations. However, in case of the redox flow battery, when operating for charging and discharging in a stopped state, it takes time until the electrolyte is circulated to the inside of the stack by the pump. In addition, the responsiveness is lowered over time, and cost is increased because a plurality of chemical resistance-pipes connecting the cell, the stack, and the pump is required.
In a typical redox flow battery, electrolyte is supplied to each cell through each manifold. However, because the electrolyte filled in the manifolds serve as an electric passage for connecting each cell, it can be an electron movement path such that a shunt current is generated through the passage. Thus, a part of the energy is lost due to the shunt current at charging and discharging processes, whereby the shunt current becomes a major cause of reduced efficiency, component damage, and uneven cell performance. In order to reduce the shunt current, increasing a length of the manifold and reducing a cross-sectional area of the manifold have been adopted. However, a flow resistance of the fluid is increased in above solutions such that pumping loss is generated, therefore, an alternative to overcome the problem is required.